1. Field of the Invention
This invention relates to liquid crystal light valve (LCLV) projectors. Specifically, this invention relates to apparatus for improving the performance of such projectors.
While the present invention will be described herein with reference to a particular embodiment, it is understood that the invention is not limited thereto. Those having ordinary skill in the art and access to the teachings of this invention will recognize additional embodiments and applications within the scope thereof.
2. Description of the Prior Art
The development of the liquid crystal light valve has opened the door to substantial progress in the state of the art of high quality large screen projectors. The reflective mode liquid crystal light valve is a thin, multi-layer structure comprising a liquid crystal layer, a dielectric mirror, a light blocking layer, and a photoresponsive sandwiched between two transparent electrodes. A polarized projection beam is directed through the liquid crystal layer onto the dielectric mirror. An input image of low intensity light, such as that generated by cathode ray tube, is applied to the photoresponsive layer thereby switching the electric field across the electrodes from the photoresponsive layer onto the liquid crystal layer to activate the liquid crystal. Linearly polarized projection light passing through the liquid crystal layer and reflecting from the dielectric mirror is polarization modulated in accordance with the information incident on the photoconductor. Therefore, if a large complex distribution of light, for example, a high resolution input image, is focused onto the photoconductive surface, the device converts the image into a replica which can be projected with magnification to produce a high brightness image on a viewing screen. U.S. Pat. No. 4,019,807 issued to D. D. Boswell et al on Apr. 26, 1977, discloses such a high performance reflective mode liquid crystal light valve.
A graphics display projector using a liquid crystal light valve of the above type is described in an article entitled "An Application of the Liquid Crystal Light Valve to a Large Screen Graphics Display", published in the 1979, SOCIETY FOR INFORMATION DISPLAY (SID), International Symposium, Digest of Technical Papers, May 1979, pp. 22-23. This display system, a type which the present invention is particularly but not exclusively concerned, projects a large scale image having yellow-white characters on a dark blue background. The invention includes a cathode ray tube (CRT) which provides input imagery; projection optics which provide the bright collimated output beam and necessary light polarization; and the liquid crystal light valve which interfaces the input and output functions.
The system uses a powerful light source such as a xenon arc lamp to illuminate the liquid crystal light valve through collimating and polarizing optics. The light emitted from the xenon arc lamp is transmitted to a main polarizing prism where it is separated into `S` and `P` components. The P component passes through the prism while the S component is reflected toward the light valve. Information displayed by the cathode ray tube is transferred by fiber optics to one side of the light valve which changes the polarization state from S to P. The light is then transmitted through the prism and imaged on the screen by a projection lens. In this capacity, the main prism functions as an analyzer, converting modulations of polarization to modulations of brightness or intensity.
As mentioned above, this system is typical of prior art liquid crystal light valve projection systems in that it projects a large scale image having yellow-white alpha numeric characters on a blue background. The colors are a result of the unavoidable fact that the liquid crystal material polarization modulates the white projection light incident upon it as a function of the wavelength of the light. Although providing an image of high brightness and resolution, the system has several inherent drawbacks. For example, since the light valve is a complicated, expensive device that includes numerous microscopic thin film layers deposited on a superflat substrate, each requires a series of critical manufacturing steps. One of which requires that the two substrates which sandwich the ultra thin liquid crystal layer be polished to an optical flatness of better than 1/4 the wavelength of white light or 0.15 micrometers. The light valves have a complicated molecular composition of rod-like liquid crystal molecules that are arranged in chains which are precisely oriented in twist and tilt angles relative to the substrates. In addition, the device must be assembled so that two optically flat substrates are uniformly separated from point to point to within a fraction of a micrometer.
The difficulty of producing such complicated structures within the required tolerances results in a large percentage of defective light valves. The variations in the liquid crystal layer thickness caused by both surface waviness and wedging of the substrate surfaces which contain the liquid crystal material, create poor uniformity of the image background color. Non-uniform twist and tilt of the liquid crystal molecules in their off-state also gives rise to color non-uniformities in the image background.
An additional cause of such uneven color is residual birefringence within the polarizing beam splitter. This residual birefringence can occur from manufacturing imperfections as well as from non-uniform heating by heat sources within the image projector package including the electronics as well as the high intensity light source.
Yet another shortcoming is due to the strong dependence of the background color on liquid crystal layer thickness. That is, the desirable color contrast of yellow characters on a blue background is achieved only by using a relatively thick liquid crystal layer of from approximately 6 to 8 micrometers. Because the response time of the liquid crystal layer varies as the square of its thickness, the blue color is achieved at the expense of frequency response. That is, as the thickness is increased to improve background, the rate at which unsmeared video images can be displayed decreases. Thus, although unsmeared video rate images can be produced with a thinner liquid crystal layer (e.g., 3 to 4 micrometers), the background may appear as an undesirable black color. This, when combined with yellow characters, gives a relatively low visually unpleasing color contrast. Finally, the blue background can vary in shade from device to device.
These defects which result in color variations, non-uniformity, and inconsistency can be redressed somewhat by using a special blue mirror to superimpose a spatially graded blue background over the light valve image. The special blue mirror will typically include a blue filter, a quarterwave plate, a spatially graded neutral density filter, and a mirrored reflective surface. A composite beam is thus projected by a lens onto a screen so as to superimpose the second beam onto the output image. The color of the second beam is selected so as the mask the color variations and improve the contrast of the output image. The graded intensity filter is selected to compensate for background brightness variations.
While the light valve operates on one channel, i.e., the S polarized light, the blue mirror operates on the other channel, i.e., the P polarized light. This, however, presents an additional difficulty. That is, the quality of the projected image, being generally a function of brightness, resolution and contrast, can be improved substantially by placing a prepolarizing prism in the optical path in front of the main polarizing prism. The prepolarizing prism substantially overcomes deficiencies in the main polarizing prism insofar that it aids in the transmission of light of one polarization and in reflection of light of another polarization. In addition, the prepolarizing prism is helpful in reducing heat generated stress in the main prism due to energy passing therethrough. (This is a problem that is exacerbated even further where a blue mirror is used insofar as the blue mirror requires and causes more energy to pass through the main prism.)
However, if the prepolarizing prism is used in the conventional manner to delete light of one polarization, the blue mirror cannot be used. This is because, as mentioned above, the blue mirror relies on the second channel to provide a second beam which enhances the projected image. Thus, it has been found desirable to provide a liquid crystal light valve projector which allows a blue mirror to be used while securing the advantages of prepolarization.